In addition to the conventional wooden pallets, plastic pallets are today playing an ever-increasing role in the transport and storage of goods. For example the lower weight and the possibility of forming almost any desired pallet structure using injection-moulding techniques are advantageous, with the result that a high degree of individuality can be achieved here, and it is possible in particular to respond to customer-specific requests. In addition, unless particular hygiene regulations are to be complied with, recycled material can be used for producing many pallet types. The use of additives such as for example reinforcing fibers is also possible. The deck can comprise a continuous, closed load platform, however the load platform can also be formed by a grid or rib structure.
On the underside of the deck, i.e. facing the ground, feet are formed protruding downwards. They have a height which makes it possible for the pallet to be picked up with the fork of a forklift truck and transported; the fork enters into the spaces between the feet. At the same time, however, the feet must also be capable of bearing the permissible weight of the pallet with goods stored thereon, without this resulting in signs of fatigue of the material. Although it is possible to produce the feet separately from a material with a higher impact strength, this type of production is more expensive in comparison with one-piece production of a pallet, as more tools have to be kept ready and the pallet then has to be assembled.
For transport on roller and chain conveyors on the one hand, and for increasing stability on the other hand, plastic pallets often also comprise runners which are formed in each case connecting at least two feet to each other on their undersides. The runners are mostly arranged parallel to each other; in the case of rectangular pallets their longitudinal direction usually lies parallel to the narrower edge of the pallet, though not necessarily: a connection of the feet along the longer edge is also possible. Circumferential runners can also be used, i.e. runners which in addition also connect the feet to each other along the longer edge of the pallet.
However, plastic pallets also have disadvantages compared with wooden or metal pallets. One disadvantage is that, under load, plastic pallets tend towards greater deformations than wooden pallets. At worst, this can lead to irreversible deformations. If goods with a high, though still permissible, mass are placed on the pallets, this leads to a deflection of the deck, wherein the feet with runners formed thereon are also slightly deformed, or bear their share of the deflection, in that the feet are inclined inwards at the top, in the direction of the deck center; however they move outwards at the bottom. Thrust, bending and shear forces thus occur, which can only insufficiently reversibly be reabsorbed by the pallet.
In order to reduce the deformation under load, it is known in the state of the art to reinforce plastic pallets with stiffening structures in order to increase in particular the bending stiffness of the pallets.
For example, DE 20 2015 100 355 U1 describes a plastic pallet that can be assembled from several parts, into the deck of which metal rods are inserted in longitudinal direction to increase the bending stiffness. The metal rods are here arranged transverse to the longitudinal direction of the runners. They reinforce the deck structure and lie parallel to each other, without being interconnected.
DE 10 2014 007 079 A1 describes a two-part plastic pallet with reinforcing profiles which have the function of stiffening elements. The stiffening elements are rod-like and are slid separately into the runners. Here the runner structure is reinforced in the area of the ground level.
In DE 10 2011 103 359 A1, FIG. 8 shows a plastic pallet in which reinforcing elements are arranged in the corners. Apart from the reinforcing elements that are not interconnected, which are also referred to as fittings, the pallet is manufactured in one piece. In the finished pallet the reinforcing elements extend from the deck to the ground and are not interconnected. Fitting the reinforcing elements exclusively in the corners serves to increase the wear resistance.
DE 10 2011 052958 A1 describes a pallet assembled from several parts, in which foot elements are formed arched and arranged crosswise. On their side facing the deck, in the area of the apex of the arches, supporting rods which can also be manufactured from metal are inserted, extending over the length of the foot elements. The bearing capacity of the pallet is increased by the grid arrangement. DE 43 36 469 A1 also describes a plastic pallet in which the deck structure is reinforced with a framework of reinforcing tubes which can for example be manufactured from steel.
DE 20 2007 000 985 U1 describes a plastic pallet which is provided with reinforcements both in the area below the deck and in the area of the feet just above the ground. According to the embodiment shown in FIGS. 1-3, the reinforcement elements which can be formed from a rod- or bar-shaped material form a grid structure in the deck, and along the narrow side of the pallet two reinforcement elements arranged one above the other lie parallel to each other, wherein one element is below the surface of the deck, embedded therein, and the other in the underside of the runner. However, the reinforcement elements are not in direct contact with each other; they are not interconnected.
WO 2007/019833 A1 describes a plastic pallet in which reinforcing elements are arranged below the base plate of the pallet in the area of the feet and within the deck. Here FIGS. 9-11 show a pallet consisting of a deck and feet attached thereto, wherein in each case three of the feet are connected in the runners along the longer side of the pallet by foot rails which can consist of steel sheet. In the deck, reinforcing elements likewise manufactured from steel sheet are arranged in the manner of a grid; the intersections of the longitudinal braces and cross braces lie in the area of the feet. There, the grid structure is connected to the foot rails via stays, wherein no more detailed statement is made about the type of connection. Polystyrene is named as preferred material for the pallet described in WO 2007/019833 A1 and the grid structure serves to increase the dimensional stability. The longitudinal and cross braces arranged in the deck as well as the stays in the feet comprise a plurality of aligned recesses, which are intended to guarantee that they can be completely penetrated by the plastic of the pallet; in this way the connection to the plastic can be improved and the stability of the overall construction can be increased compared with that of a simple polystyrene pallet. In addition, the high number of recesses ensures that the weight of the pallet does not increase excessively compared with that of a pure polystyrene pallet.
Although such a structure of stiffening elements with recesses is very advantageous with respect to the weight and the connection to the plastic, and increases the stability with respect to direct loading from above, there is scarcely any increase in loading due to shear forces. In addition, the connection of the longitudinal or cross braces to the foot rails via the stays occurs only through the bond in the plastic, with the result that the pallet can withstand only low bending and shear forces.